The instant invention relates to a graft polymer with ethylenically unsaturated, polymerizable or crosslinkable lateral chains and to a photocurable mixture which can be developed with aqueous solutions and in which said graft polymer and a photosensitive substance are contained. The mixture is suited for the production of photosensitive recording materials, in particular of planographic printing plates and photoresists.
From U.S. Pat. No. 2,929,710, GB-A 834,337 and DE-A 15 22 359, the use of polyvinyl alcohol derivatives with lateral polymerizable double bonds in photopolymerizable mixtures is known. These derivatives of polyvinyl alcohol with polymerizable groups are, for example, unsaturated polyvinyl acetals or unsaturated polyvinyl esters. Due to their relatively high content of impurities, the polyvinyl alcohol derivatives described in the above publications are suited for use in photopolymerizable recording materials to a limited extent only. Moreover, the vinyl alcohol units have been reacted to such an extent that organic solvents are required for developing the imagewise exposed layers, and hence these layers do not meet to-date demands.
Polyvinyl alcohol derivatives containing just a small proportion of reacted vinyl alcohol units are used for the printing plates in accordance with GB-A 834,337, U.S. Pat. No. 3,877,939,U.S. Pat. No. 4,247,624, DE-A 31 28 949, DE-A 31 44 905 and DE-A 33 22 994, which can be developed with aqueous solutions. The printing forms are relatively brittle and thus give low print runs. The hydrophilic properties of the binders, which are required to achieve developability with aqueous media, are maintained even after the crosslinking reaction. This leads to swelling during development and to poor stability during the subsequent rinsing step. In most cases, it is very difficult to reproduce fine image elements with these recording materials.
The known recording materials, which can be developed with aqueous solutions, furthermore tend to the formation of residual layers during development, i.e., those portions of the recording layer, which have not been polymerized or crosslinked upon imagewise exposure, cannot be removed completely from the support, and layer residues are left behind. This phenomenon, which is in particular observed when metallic supports are used for the photocurable recording layer, is of disadvantage above all in the production of resist patterns and planographic printing plates.
EP-A 0,248,424 and EP-A 0,248,395 disclose photopolymerizable recording materials which can be washed out with aqueous-alkaline developer solutions and are suitable for producing printing plates. The binders contained in these materials are carboxyl-containing copolymers where COOH groups have been reacted by esterification with glycidyl (meth)acrylate. This modification results, on the one hand, in a reduction of the acid number of the binder, which renders the plate less easily developable, and, on the other hand, in a reduced glass transition temperature of the binder, which negatively affects the print run.
In DE-A 37 32 089 graft polymers are described, which comprise a polyurethane as the graft backbone and grafted-on vinyl ester units, which are at least partially saponified to give vinyl alcohol units. The polymers are suitable as binders for pigments; for preparing printing inks, thermoplastic adhesives and solvent-containing adhesives; as constituents of varnishes or coatings for fibers, films and metals, and for thermoplastic shaped articles.
Prior German Patent Application P 38 35 840.9 describes graft polymers which are obtained from graft polymers described in DE-A 37 32 089 by acetalization with saturated aliphatic or aromatic aldehydes.
Prior German Patent Application P 39 20 229.1 describes photopolymerizable mixtures in which the graft polyvinyl acetals described in P 38 35 840.9 are contained as binders.